The pairing of homologous chromosomes during meiosis is a feature displayed by nearly all sexually reproducing organisms. Surprisingly, little is known about the way these homologous chromosomes recognize each other, or the way once recognized, they are packaged into a pairing structure known as the synaptonemal complex. The long-term goal of this project is an exploration into the structure of the synaptonemal complex. The approach begins with the yeast protein HOP1. By genetic and cytological criteria, this protein appears to be a structural component of the yeast synaptonemal complex. The first part of the proposal will test the hypothesis that HOP1 protein directly contacts chromosomes and perhaps even functions to correctly pair homologues. The second - - and longer term - - part of the proposal is designed to identify other components of the synaptonemal complex. Because we expect that this structure is formed in part by strong protein-protein interactions, we have chosen to use HOP1 as an affinity reagent to identify proteins with which it interacts. The eventual goal of the project is to identify the structural components of the synaptonemal complex and to learn how these proteins interact with themselves and with the chromosomes to assemble this remarkable structure. Understanding how cells accurately segregate chromosomes should aid in understanding how the process fails, an event that gives rise to several serious medical problems, for example, Down's Syndrome.